LEDs have advantages of having higher efficiency and longer life compared to fluorescent lamps, which have conventionally been used as illumination. In recent years, the LEDs have had higher brightness and have been used as a source of illuminating light for saving energy. For example, the LEDs are used as illumination for traffic signals.
Furthermore, the LEDs have a very high response speed and can be electrically controlled. By using these characteristics, studies about communication using LEDs have been performed recently. In the communication using LEDs, the LEDs can be used as illuminating light because human's eyes cannot perceive very high-speed blinking of the LEDs.
Typically, a plurality of LEDs are used as a source of illuminating light because only one LED has insufficient brightness. In communication using illuminating light, data is transmitted by OOK (on-off-keying) modulation or the like in the plurality of LEDs provided in a transmitter, and the data is received by a photodetector provided in a receiver. However, a maximum data rate obtained in this method is determined depending on the performance of the LEDs, such as a response speed, and another mechanism is required to increase the data rate.
Under these circumstances, various suggestions to increase the data rate have been made. For example, Japanese Unexamined Patent Application Publication No. 2001-292107 disclosed the following technique. That is, LEDs are arranged in a matrix pattern of (1,1) to (N,N) and serial data is converted to parallel of N×N in a transmitter. Data is received by photodiodes in a receiver, serving as photoreceivers placed in a matrix pattern, from respective elements by using a lens, so as to increase the data rate. In this case, the brightness can be increased by using a set of 100 LEDs for each matrix element. For example, assume that each element including the set of LEDs enables a data rate of 10 Mbps. In that case, if the number of elements is 4×4=16, a data rate of 160 Mbps can be realized in total.
In the above-described technique, serial data is mapped in parallel by using a predetermined placement of a light source and parallel communication is performed to increase the data rate to N times that of serial communication.